configs: stm32mp15: increase the number of reserved memory region in lmb

For the latest kernel device tree the max number of reserved regions
in lmb library is reached: 8 with 5 reserved regions in device tree.

When a new region is added, the lmb allocation for the device tree
relocation failed and boot with ramdisk failed.

This patch avoids this issue by increasing the max number of
supported reserved memory in lmb library to 16.

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>